A settled summer on the cards

NATIONAL CLIMATE CENTRE 31 October 2008

SEASONAL CLIMATE OUTLOOK: November 2008 – January 2009

A settled summer on the cards

The NIWA National Climate Centre says that much of this coming summer is likely to be relatively warm, settled, with rainfalls near normal or below normal overall.

The centre’s seasonal climate outlook for November 2008 to January 2009 indicates that above average temperatures are the most likely outcome across most of the country. Normal or below normal rainfalls are the most likely outcome for the three months as a whole, though there will inevitably be wet periods during that time. Normal or below normal soil moisture levels and stream flows are likely in most regions, apart from the east of the South Island where below normal conditions are likely.

Mean sea-level pressures are likely to be higher than normal over, and to the east of the country, with lighter winds than normal in many places.

There is presently no La Niña or El Niño in the tropical Pacific, and no indication of either La Niña or El Niño developing over the summer. The chance of an ex-tropical cyclone affecting New Zealand over the summer is near the long-term average.

Overall picture:
Temperature:
Air temperatures are likely to be above average in most regions of the country, but near average in the eastern South Island. Sea surface temperatures around New Zealand are expected to be near normal.

Rainfall, soil moisture, and stream flows:
Rainfall is likely to be near normal in the north and east of the North Island, and normal or below normal elsewhere. Normal or below normal soil moisture levels and stream flows are likely in most regions, apart from the east of the South Island where below normal conditions are likely.

[Reporters please note: Probabilities are assigned in THREE categories; above average, average, and below average. See end for more explanation.]

Regional predictions for the next three months:
Northland, Auckland, Waikato, Bay of Plenty:
Above average temperatures are likely. Rainfall is likely to be near normal, with soil moisture and stream flows likely to be normal or below normal for the season as a whole.

Central North Island, Taranaki, Wanganui, Manawatu and Wellington:
Above average temperatures are likely overall. Normal or below normal rainfall and stream flows are likely, with near normal soil moisture levels.

Gisborne, Hawkes Bay, Wairarapa:
Above average temperatures are likely for the three months November-January. Normal rainfall is likely overall, with normal or below normal soil moisture and stream flows.

Nelson, Marlborough, Buller:
Above average temperatures are likely, while normal or below normal rainfalls, soil moisture levels and stream flows are likely for the three-month average.

West Coast, Alps and Foothills, Inland Otago, Southland:
Above average temperatures are likely. Rainfall, soil moisture, and stream flow, are all likely to be normal or below normal.

Coastal Canterbury, East Otago:
Average temperatures are likely for the early summer period. Rainfall is likely to be normal or below normal, while soil moisture and stream flows are likely to be below normal.

Background

Climate and Oceans:
In the New Zealand region, mean sea level pressures are likely to be higher than normal, especially east of the country, resulting in lighter winds than usual over most regions.
Neutral conditions (no El Niño or La Niña) now prevail in the tropical Pacific and are very likely to continue through the summer. While the tropical Pacific Ocean is close to a neutral state, the Southern Oscillation Index remains positive and stronger than normal easterly trade winds are present in the western equatorial Pacific. These atmospheric signs of La Niña are expected to ease towards normal through the next few months. All climate forecasting models indicate conditions in the neutral range during November to January and beyond.
The tropical cyclone season for the southwest Pacific begins in November, and with ENSO neutral conditions in the Pacific, there is a normal (4 out of 5) chance of an ex-tropical cyclone passing within 500 km of the country between November and May, with the districts at highest risk being Northland and Gisborne.

© Copyright NIWA 2008. All rights reserved. Acknowledgement of NIWA as the source is required.

Notes to reporters & editors

1. NIWA’s outlooks indicate the likelihood of climate conditions being at, above, or below average for the season as a whole. They are not ‘weather forecasts’. It is not possible to forecast precise weather conditions three months ahead of time.

2. The outlooks are the result of the expert judgment of NIWA’s climate scientists. They take into account observations of atmospheric and ocean conditions and output from global and local climate models. The presence of El Niño or La Niña conditions and the sea surface temperatures around New Zealand can be a useful indicator of likely overall climate conditions for a season.

3. The outlooks state the probability for above average conditions, average conditions, and below average conditions for rainfall, temperature, soil moisture, and stream flows. For example, for winter (June-July-August) 2007, for all the North Island, we assigned the following probabilities for temperature:
 Above average: 60%
 Average: 30%
 Below average: 10%
We therefore conclude that above average temperatures were very likely.

4. This three-way probability means that a random choice would only be correct 33% (or one-third) of the time. It would be like randomly throwing a dart at a board divided into 3 equal parts, or throwing a dice with three numbers on it. An analogy with coin tossing (a two-way probability) is not correct.

5. A 50% ‘hit rate’ is substantially better than guess-work, and comparable with the skill level of the best overseas climate outlooks. See, for example, analysis of global outlooks issued by the International Research Institute for Climate and Society based in the U.S. (http://iri.ldeo.columbia.edu/) published in the Bulletin of the American Meteorological Society (Goddard, L., A. G. Barnston, and S. J. Mason, 2003: Evaluation of the IRI's “net assessment” seasonal climate forecasts 1997-2001. Bull. Amer. Meteor. Soc., 84, 1761-1781).

6. Each month NIWA publishes an analysis of how well its outlooks perform. This is available on-line and is sent to about 3,500 recipients of NIWA’s newsletters, including many farmers. See The Climate Update: www.niwascience.co.nz/ncc

7. All outlooks are for the three months as a whole. There will inevitably be wet and dry days, hot and cold days, within a season.

8. The seasonal climate outlooks are an output of a scientific research programme, supplemented by NIWA’s Capability Funding. NIWA does not have a government contract to produce these outlooks.

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